Wet strength is a desirable attribute of many disposable paper products that are required to maintain their wet integrity for an extended period of time prior to and during their intended use. Such products include toilet tissue, diapers, personal care products, and pre-moistened articles such as baby wipes and household cleaning wipes.
Permanent wet strength, however, is often an unnecessary and undesirable characteristic in paper products. Due to the permanent wet strength of such products, paper products are generally disposed of after brief periods of use into landfills, incinerators, etc. and thus can pose a significant burden on the solid waste stream. It is therefore preferable to direct used paper products to municipal sewage treatment facilities or private septic systems. Unfortunately, this procedure is often not possible if the product is “unflushable.” Clogging of sewage and septic systems can result if the product permanently retains hydrolysis-resistant strength properties. To address this problem, efforts have been undertaken to provide binders that will provide paper products with sufficient “temporary” wet integrity in the presence of saline solutions, but minimal integrity when exposed to large amounts of waste water, such that they traverse plumbing and disintegrate in sewage and/or septic systems.
Specifically, it has been attempted to produce disposable fibrous products which maintain a relatively high wet strength in the presence of solutions with elevated ion concentrations, but become more dispersible when in contact with solutions having a lower ion concentration. These ion-sensitive, water dispersible polymer formulations are well known in the art. There is disclosed for example, in U.S. Pat. No. 6,429,261 to Lang et al. a polymer formulation comprising a triggerable copolymer of acrylic acid, NaAMPS, butyl acrylate and 2-ethylhexyl acrylate as well as a non-crosslinkable co-binder polymer comprising poly(ethylene-vinyl acetate), wherein the polymer formulation is insoluble in a neutral salt solution containing at least about 1 weight percent salt and wherein the triggerable copolymer is soluble in water containing up to about 200 ppm of one or more multivalent ions.
U.S. Pat. No. 6,291,372 of Mumick et al. discloses ion sensitive binders for fibrous material. A water soluble polymer binder for binding a fibrous substrate includes about 25 to about 85 weight percent of unsaturated carboxylic acid and ester terpolymer; as well as from about 5 weight percent to about 35 weight percent of a divalent ion inhibitor and from about 10 weight percent to about 60 weight percent of a hydrophilic cross-linkable polymer. The polymer binder is useful for binding absorbent webs of the class used in personal care products such as pre-moistened wipes.
U.S. Pat. No. 5,631,317 to Komatsu et al. discloses a process for producing self-dispersing and salt-sensitive polymer. The formulations include from a) about 35 to about 65 weight percent of acrylic acid; b) from about 10 to about 45 weight percent of an acrylic ester such as 2-ethylhexyl(meth)acrylate or lauryl(meth)acrylate; and c) from about 20 to about 45 weight percent of a second acrylic ester such as ethyl (meth) acrylate, isopropyl(meth)acrylate, or n-butyl(meth)acrylate.
U.S. Pat. No. 4,278,727 to Brabetz et al. discloses a binder emulsion for non-woven materials that comprises protective colloids, optional emulsifiers, and a copolymer that incorporates monomers of vinyl acetate, a vinyl ester of higher alkanoic acid and/or higher alkyl esters of acrylic or methacrylic acids, and an ethylenically-unsaturated acid. The binder formulations in Brabetz, however, are reported to be relatively water resistant, but soluble in basic solutions. The binders impart a relatively high wet strength to fiber when soaked in water and do not appear to be salt-sensitive.
Still further references of interest with respect to salt sensitive binders include the following: U.S. Pat. No. 6,683,129 to Eknoian which discloses salt sensitive binders in aqueous emulsions; U.S. Pat. No. 5,312,883 to Komatsu et al. which discloses water soluble salt sensitive polymers; and U.S. Pat. No. 6,127,593 to Bjorkquist et al. for flushable fibrous structures.
While salt sensitive binders are known, there exists a need for salt sensitive binders which are readily produced from available low-cost monomers and which form stable solutions that are easy to apply during web manufacture.